Polyethylene naphthalate, one of the polyesters that have been broadly used in the art, is possessed with excellent physical and chemical properties such as heat resistance, durability, chemical resistance, radiation resistance and electrical insulation capability as well as good mechanical properties such as tensile strength, elasticity and impact strength; and, therefore, has been used for preparing various articles including magnetic recording media, capacitors, wrapping materials, photographic film and other commercial applications.
Particularly, biaxially oriented film of polyethylene naphthalate is proved to provide better mechanical strength, heat stability and dimension stability than polyethylene terephthalate film which has been widely used as a base film in the magnetic recording medium. Owing to such advantageous properties, polyethylene naphthalate film is being used as a carrier for 8 mm tapes, type C VHS tapes and video tapes with longer recording time.
Further, polyethylene naphthalate can be effectively sterilized due to its superior heat resistance (Tg=115.degree. C.) and, due to its good gas-barrier property, may be used in manufacturing bottles and other packaging materials.
In order to prepare polyethylene naphthalate possessed with good mechanical strength and heat stability, it is required to have a high intrinsic viscosity, e.g., greater than 0.4 dl/g. However, should the intrinsic viscosity exceed 100,000 centipoise (at 295.degree. C., shear 100 sec.sup.-1), there may arise various problems including the overloading of the motor for stirring the reactants during the polycondensation or extrusion molding process, which may result in the introduction of undesirable "bubbles" into the product in addition to other handling and reaction control problems.
If the bubbles containing ethylene glycol, used as a starting material, and acetaldehyde, cyclic oligomers and water components, generated as reaction by-products, are introduced into the polymer product, it may significantly lower the intrinsic viscosity of the polymer, thereby deteriorating the mechanical, thermal and chemical properties of the film obtained therefrom.
To solve the above-mentioned problems, various attempts have been made. For instance, a process for removing the bubbles incorporated into the polymer by way of alternatively applying an elevated pressure and a reduced pressure in a reactor after the polycondensation process has been completed is described in Japanese Laid-open Patent Publication No. 260822/1987. However, this method tends to damage the polymer, if repeated.
Further, a process for removing such bubbles during the extrusion process by way of employing an extruder equipped with a ventilation funnel is known to those skilled in the art. However, deterioration of the polymer can still occur before such debubbling process is carried out.